The broad goal of this proposal is to understand the mechanisms controlling blood cell production by studying circumstances in which erythropoiesis is perturbed. Feline leukemia virus, subgroup C/Sarma (FeLV-C/Sarma) (and not FeLV, subgroups A or B) causes pure red cell aplasia (PRCA) in cats. These viruses differ in discrete regions of the envelope glycoprotein gp70 (designated variable regions [VR], 1-4), and the genetic determinant of anemia has been localized to the 30 amino acid N-terminal hydrophilic VR1 region. This disease does not have a neoplastic or immunologic origin. BFU-E persist in marrow cultures from cats with PRCA, yet CFU-E are not detected suggesting that the gp70 of FeLV-C/Sarma selectively impairs the differentiation of BFU-E to CFU-E. The aim of this proposal is to determine the mechanisms of this lineage and stage- dependent defect. VR1 of FeLV-C/Sarma is also responsible for the unique host cell range of subgroup C viruses (i.e., these viruses infect human and guinea pig, but not hamster cells). This implies that the VR1 region is required for receptor binding, and by extension, that PRCA might result from the improper expression or function of the cell surface for FeLV- C/Sarma and/or of a cytokine receptor that is needed for early erythropoiesis. We will try to identify the receptor for FeLV-C/Sarma by first determining its chromosomal localization, adapting a strategy used in studies of the amphotropic murine leukemia virus cell surface receptor (J. Virol. 65:6316, 1991). These results will be used to direct expression cloning experiments (if required). Next we will optimize conditions for the suspension culture of feline (and human) marrow cells and determine if FeLV-C/Sarma env protein (or intact virus) is responsible for defective erythropoiesis in vitro . We will determine if defects of CFU-E maturation in FeLV-C/Sarma-infected cultures reverse when serum components or specific cytokine(s) (erythropoietin, SCF, IL-3, G-CSF, or GM-CSF, etc.) are added. We will also study how FeLV-C/Sarma infection influences the growth of growth factor-dependent human cell lines (e.g. TF-1) and the erythroid differentiation of K562, MB-02, or HEL cells. As a preliminary study, we have determined that FeLV-C/Sarma-infected K562 cells fail to hemoglobinize after exposure to butyrate or other inducing agents. When expressing high titer gp70, K562 cells also fail to grow. We will determine if this growth arrest results from apoptosis. In final studies, we will correlate in vitro observations with the progression of anemia in viremic cats. Taken together, these studies should define the pathophysiology of feline PRCA.